Evidence indicates that certain animals detect and orient themselves to the earth's magnetic field. This capacity may be used in the context of homing, finding mates, and locating food sources, for instance. Although the existence of biosensory apparatus for transducing geomagnetism can be inferred from the orienting responses of diverse animals, little is yet known of the mechanisms of transduction of geomagnetic energy into the nervous system of any multicellular animal. An animal has been found which has two unusual features and which provides an opportunity to achieve an understanding of the physical/biological basis for the geomagnetic sense. The marine nudibranch mollusc, Tritonia diomedia response to the earth's magnetic field by orienting appropriately when other cues are unavailable. Secondly, in recent experiments, a symmetrical pair of re-identifiable neurons in the animal's brain have been found which respond to changes in the direction of the ambient magnetic field by firing electrical impulses. In this proposal, experiments are described at the behavioral, neuronal circuitry, and single neuron level that are designed to (i) establish the tuning of the geomagnetic sensory system at the behavioral level and in appropriate re-identifiable neurons, (ii) trace further the neural circuitry that couples this sensory input to the relevant motor control using electrophysiological techniques, and (iii) determine cellular and molecular level mechanisms underlying the transduction of magnetic energy into nervous system signals.